Reraise existing exception if dtype ctor fails

include/pybind11/numpy.h

@@ -189,7 +189,7 @@ public:
     static dtype from_args(object args) {
         PyObject *ptr = nullptr;
         if (!detail::npy_api::get().PyArray_DescrConverter_(args.release().ptr(), &ptr) || !ptr)
-            pybind11_fail("NumPy: failed to create structured dtype");
+            throw error_already_set();
         return object(ptr, false);
     }